In conventional technologies, either synchronous or asynchronous Hybrid Automatic Repeat Request (HARQ) are used, with a certain number of HARQ processes. It is usually assumed that every transmission using HARQ is scheduled by a network node, such as an enhanced node B (eNodeB). However in fifth generation (5G) radio access, also contention based data transmissions may benefit from novel HARQ solutions tailored for low latencies with minimal control signalling.
In contention based data transmission, a User Equipment (UE) may access shared radio resources at any time without any scheduling. Therefore collisions will happen, but no signalling is required for scheduling. It would be desirable to send retransmissions in contention based manner without HARQ synchronisation problems.
In for example Third Generation Partnership Program (3GPP) Long Term Evolution (LTE), asynchronous Type-II HARQ transmission is used on the downlink. This means the receiver does not know ahead of time what is being transmitted, or when, so the HARQ process identifier and the Redundancy Version (RV) must be sent along with the data. The RV specifies which combination of data, Error-Detecting (ED), and Forward Error Correction (FEC) bits is being sent to the UE.
This is done through the Physical Downlink Shared Channel (PDSCH) resource allocation messages sent on a Physical Downlink Control Channel (PDCCH) simultaneous to the corresponding PDSCH transmission. The advantage of this scheme is that the scheduling algorithm has considerable freedom in deciding which UEs are sent data during any subframe.
In contrast, LTE uses synchronous HARQ transmission on the uplink (UL). This means that the eNodeB knows exactly which HARQ process and RV the UE will transmit ahead of time. So, this information does not have to be comprised in the PDCCH message providing the uplink scheduling information to the UE.
Synchronous HARQ can be used because the UE transmits the same HARQ process every eighth subframe. Because retransmissions of a HARQ process are associated with previous transmissions based on the eight-subframe delay, the scheduling in the uplink is not quite as flexible as that in the downlink.
Uplink retransmissions may be either adaptive or non-adaptive. When the UE receives a Negative Acknowledgement (NACK), then the UE shall send non-adaptive retransmission in subframe n+4 utilising the same resources which were allocated for the initial transmission. Adaptive retransmission is done when uplink grant is received and New Data Indication (NDI) bit is not toggled. In case of adaptive retransmission request, the UE does not care about HARQ feedback received for previous transmission attempt.
There are technical problems that may occur when any known conventional solution is applied in a 5G dense Radio Access Technology (RAT) system where latencies has to be <1 ms and control overhead must be minimised to make things efficient and also as simple as possible.
Asynchronous HARQ requires signalling between the involved nodes, since the HARQ process ID and RV information have to be carried for every transmission. Additionally non-adaptive asynchronous retransmissions are not possible.
Synchronous HARQ does not need HARQ process ID and RV information in order to work robustly, but it does not give as much scheduling freedom to network. This is a significant problem when a new scheduling dimension in Multi User (MU)—Multiple-Input and Multiple-Output (MIMO) and Coordinated Multipoint (CoMP) has to be taken into account. When UEs are scheduled also spatially in addition to time and frequency domains, UE orthogonality in spatial domain must be taken into account, which limits possible scheduling combinations for each Transmission Time Interval (TTI).
Conventional HARQ solutions do not take contention based data transmissions well into account, because an underlying assumption is that transmissions using HARQ are always scheduled in a way or another. It has been envisioned that one significant improvement in 5G compared to legacy systems is to support Machine-to-Machine (M2M) type of communication. Efficient way to do this is contention based data transmissions for small packets. With contention based transmissions there will not be UL grants, hence HARQ process IDs cannot be indicated for contention based UL data. If HARQ is still required, solution to handle also retransmissions on contention based manner is needed if the UE did not receive positive acknowledgement for the initial transmission attempt.
In 5G dense deployment supporting high UE density and mobility there will be many radio units operating close to each other and using coordinated beamforming techniques. Additionally more than one access nodes may send data jointly to any single UE. Hence, it would be beneficial that retransmission timing is not fixed to achieve more scheduling freedom and HARQ processes would not need to be synchronised between single access node and UE.
Hence, it is a general problem to assure that there is a reasonable trade-off between control channel overhead and performance when transmitting HARQ information.